The present invention relates to an electronic blood pressure meter capable of performing an automatic pressuring operation according to the blood pressure of a subject and measuring the blood pressure rapidly with accuracy.
An electronic blood pressure meter includes a cuff, a pump for pressuring the cuff, an evacuation valve for evacuating the cuff, a pressure sensor for detecting a cuff pressure, and a microcomputer (MPU).
The MPU includes the functions of calculating a pulse wave amplitude value from the pulse wave component detected from an output signal of the pressure sensor, and determining a maximum blood pressure value (SYS) and a minimum blood pressure value (DIA) from the cuff pressure and the pulse wave amplitude value. The electronic blood pressure meter generally adopts a threshold technique in determining the blood pressure value. When making measurements, the cuff is pressured to check the flow of blood in the artery, and then pulse wave amplitude values incorporated in the cuff pressure are detected during the evacuation process. The pulse wave is a volumetric variation of the artery caused when the blood starts flowing during the process of evacuating the cuff at a slow speed. This volumetric variation is transmitted to the cuff and thus detected. The pulse wave amplitude value depicts a curve (an envelope) that gradually increases as the cuff pressure decreases, reaches a maximum, and then tends to decrease temporarily. Here, the maximum pulse wave amplitude value is detected, and during the phase in which the pulse wave amplitude increases, the pulse wave amplitude value closest to the threshold, which is the predetermined percentage of the maximum pulse wave amplitude value (e.g., 50% of the maximum pulse wave amplitude value), is detected, and the maximum blood pressure value is set to the cuff pressure at this point. Further, during the phase in which the pulse wave amplitude decreases, the pulse wave amplitude value closest to another threshold, which is a predetermined percentage of the maximum pulse wave amplitude value (e.g., 70% of the maximum pulse wave amplitude value), is detected, and the minimum blood pressure value is set to the cuff pressure at this point.
The electronic blood pressure meter pressures the cuff to a target pressuring value (a pressuring value above the maximum blood pressure value) to stop the flow of blood in the artery, obtains thereafter a cuff pressure and pulse wave information during the process of evacuating the cuff at a predetermined low speed (4 mmHg/s), and determines a blood pressure based on the cuff pressure and the pulse wave information.
Oscillometric electronic blood pressure meters track a pulse wave superposed on a cuff pressure and calculate a blood pressure based on a change in the amplitude of the pulse wave. The amplitude is, as shown in FIG. 1, small when the cuff pressure is sufficiently large with respect to a systolic pressure (a maximum blood pressure), increases as the cuff is being evacuated thereafter, and maximizes immediately before the cuff pressure becomes equal to a diastolic pressure (a minimum blood pressure). It is assumed that the cuff pressure becomes substantially equal to the average blood pressure. It maximizes at a maximum pulse wave point P. The amplitude decreases thereafter. The blood pressure is calculated based on this change in amplitude.
In an oscillometric method in which the blood pressure is calculated based on this amplitude variation, an entire amplitude change pattern (an envelope) is required. For example, according to a technique, a systolic pressure (the maximum blood pressure) and a diastolic pressure (the minimum blood pressure) are calculated as points S and D at which the pulse waves become equal to certain values on the higher pressure side and on the lower pressure side than the maximum points (points P), respectively, the certain values being obtained by multiplying the maximum values by certain percentages. More specifically, the blood pressure cannot be calculated unless all the points, S, P, and D, on the envelope are tracked. To track these three points means, it is necessary to pressure the cuff to a value larger than the point S before starting a measuring operation. If a cuff pressure larger than the point S is not obtained, the absence of the point S is discovered when the point P is detected in the blood pressure measurement operation, which makes the measurement unsuccessful due to failure to calculate the systolic pressure (the maximum blood pressure). Therefore, the user must repeat the pressuring operation at a pressure higher than in the last pressuring operation and make a measurement again. Since the point S (systolic pressure) is unknown to the user and it is very difficult to set the pressuring operation properly by estimating this point when a subject is has hypertension causing the blood pressure to undergo a noticeable variation. Further, if a measurement has been unsuccessful, it takes time to complete the measurement. Hence, a conventional practice is that the pressuring value is set to a point higher than necessary, which often keeps a subject under pain disadvantageously. For these reasons, a function of automatically setting the pressuring value has been called for.
In the above-mentioned conventional electronic blood pressure meter, the pulse wave amplitude is tracked during the cuff pressuring process in which the cuff is being pressured for a predetermined time and at a predetermined pressuring speed as shown in FIG. 2. If a plurality of pulse wave amplitudes sufficient to allow a maximum blood pressure to be estimated, can be obtained during the pressuring process, a maximum blood pressure can be estimated by applying a predetermined algorithm, and by stopping the cuff pressuring operation when the cuff pressure reaches an ideal pressuring target. An ideal pressuring target can be obtained by adding a predetermined pressure to the estimated maximum blood pressure. However, as shown in FIG. 3, for example, if an arm is thin, the cuff pressuring speed may become too fast. In such a case, if the pressuring speed is not variable as in the conventional system, a part of the pulse wave is missed no matter how many times the cuff pressuring operations are repeated. As a result, pulse wave information needed to determine a maximum blood pressure cannot be obtained. Thus, no maximum blood pressure is estimated. Therefore, the disadvantage that no proper pressuring target can be determined exists.
Since the predetermined slow evacuation is effected during a measurement in this blood pressure measuring system, it takes time to complete the measurement. As a result, the subject must endure pressure from the cuff for a long time, and the measurement may be painful to the subject. Particularly, a hypertensive subject must endure a slow evacuation process in which a large pressure is applied to the cuff. The evacuation process is so long that the subject feels numbness in his arm or gets some congestion at the artery, etc.